1. Field of Invention
The invention relates to charge coupled image sensor devices, and in particular, the invention relates to a new hydrogen diffusion path through such a device to a silicon surface to passivate surface states on the silicon surface.
2. Description of Related Art
The invention relates to a charge-coupled image sensor comprising a silicon body with a surface, parallel channel regions, formed in said body, and channel stop regions mutually separating these regions being adjacent to said surface, which surface is provided with a gate dielectric composed of a layer of silicon oxide covered with a layer of silicon nitride on which gate electrodes of polycrystalline silicon are formed so as to extend transversely to the channel regions and channel stop regions, at least a number of said gate electrodes being provided with a top layer of silicon nitride and extending into diffusion windows, at the location where said gate electrodes cross channel stop regions, which diffusion windows are formed in the layer of silicon nitride of the gate dielectric, an insulation layer of silicon oxide being provided on all gate electrodes, and, on top of said insulation layer, shunt electrodes of polycrystalline silicon situated above the channel stop regions being formed extending in a pattern of contact windows formed in the insulation layer, within which gate electrodes are exposed.
Such an image sensor, which can be used to record television images or digital photographs, comprises, in practice, several million picture elements. Each picture element is formed by a part of a channel region extending below a number, generally four, juxtaposed gate electrodes. Said gate electrodes are clustered in groups of four juxtaposed electrodes G1, G2, G3 and G4, the gate electrodes G1 of these groups being connected to each other as are the gate electrodes G2, G3 and G4. During recording an image, the voltages applied to the gate electrodes are such that charge is stored in the picture elements. The charge packets thus collected are transported through the channel regions to a read-out register integrated on the silicon body by successively applying suitable clock pulses to the gate electrodes. The thin, narrow polycrystalline gate electrodes are comparatively long, as a result of which their electrical resistance is comparatively high. For this reason, polycrystalline silicon shunt electrodes extending transversely to the gate electrodes are applied. Said shunt electrodes contact the gate electrodes in accordance with a pattern. Said shunt electrodes are also clustered in groups of four juxtaposed electrodes S1, S2, S3 and S4. The shunt electrodes S1, S2, S3 and S4 are then connected to, respectively, the gate electrodes G1, G2, G3 and G4. To reduce the resistance of the shunt electrodes, said shunt electrodes may be provided with a top layer composed of a layer of titanium, a layer of titanium nitride and a layer of tungsten.
H. L. Peek et al. disclose in xe2x80x9cA Low Current Double Membrane Poly-Si FT Technology for CCD Imagersxe2x80x9d, IEDM Techn. Digest, p. 871, 1999, particularly in FIG. 5 and the associated description, an image sensor of the type described in the opening paragraph, wherein the gate electrodes are formed in two layers of polycrystalline silicon. A number of gate electrodes, forming a first system of gate electrodes, is formed in a first layer of polycrystalline silicon. This first system is provided with a top layer of silicon nitride. Subsequently, a second system of gate electrodes is formed, between these gate electrodes, in a second layer of polycrystalline silicon. This system is not provided with a top layer of silicon nitride. The gate electrodes of the first system extend into diffusion windows formed in the silicon nitride layer of the gate dielectric at locations where the gate electrodes cross the channel stop regions. A continuous layer of silicon nitride is situated below the gate electrodes of the second system. Said diffusion windows are provided to enable passivation of surface states between the silicon body and the gate dielectric by means of a thermal treatment in hydrogen. In such a thermal treatment, hydrogen readily diffuses through polycrystalline silicon and through silicon oxide, whereas silicon nitride is impermeable to hydrogen. However, hydrogen is capable of penetrating the layer of silicon oxide of the gate dielectric and hence reach said surface states through the diffusion windows provided in the silicon nitride layer that is impermeable to hydrogen.
The above-mentioned article shows that such a passivation of surface states does not always yield the desired results. It has been found that this is caused by the fact that hydrogen cannot always reach the gate electrodes of the first system below which the diffusion windows are formed. In the polycrystalline silicon, the diffusion of hydrogen takes place along grain boundaries. In the comparatively thin, comparatively lightly doped gate electrodes, there are enough grain boundaries to enable diffusion of hydrogen through the diffusion windows in the silicon oxide layer of the gate dielectric. During forming the comparatively thick and comparatively heavily doped shunt electrodes, large silicon crystals can be formed in the contact windows. In the contact windows etched through the silicon nitride top layer of the first system of gate electrodes, said large silicon crystals can close the opening in the top layer of silicon nitride. As a result thereof, there are no grain boundaries inside the opening in the silicon nitride top layer, so that the transport of hydrogen is blocked. During operation of the sensor, diamond-shaped image errors occur. To preclude said image errors, it is proposed to remove the silicon nitride top layer from the first system of gate electrodes before depositing the layer of silicon oxide.
A drawback of this measure resides in that the light sensitivity of this sensor is smaller than that of a sensor whose silicon nitride top layer has not been removed. The silicon nitride layer below the silicon oxide layer can counteract the reflection of light, enabling the light sensitivity to be increased by maximally 20%.
It is an object of the invention, inter alia, to provide a sensor wherein said drawback is precluded.
To achieve this, the image sensor mentioned in the opening paragraph is characterized in accordance with the invention in that windows are formed in the silicon nitride top layer present on a number of gate electrodes, which windows are in line with the diffusion windows in the silicon nitride layer of the gate dielectric, a number of said windows forming part of contact windows wherein the shunt electrodes of polycrystalline silicon extend, while the other windows are filled with the silicon oxide of the insulation layer.
The windows forming part of the contact windows wherein the shunt electrodes of polycrystalline silicon extend can be closed to hydrogen transport by silicon crystals. The other windows are filled with silicon oxide of the insulation layer and will remain xe2x80x9copenxe2x80x9d to hydrogen. Hydrogen can reach the gate electrode through the silicon oxide in these xe2x80x9copenxe2x80x9d windows and subsequently the layer of silicon oxide of the gate dielectric through the subjacent diffusion windows. The xe2x80x9copenxe2x80x9d windows are arranged above the channel stop regions and hence between the actual picture elements. Surprisingly, it has been found that the provision, between the picture elements, of comparatively small xe2x80x9copenxe2x80x9d windows in the silicon nitride top layer enables said diamond-shaped image errors to be precluded without adversely affecting the light sensitivity of the image sensor.
An even greater light sensitivity is obtained if, apart from the gate electrodes extending into said diffusion windows, other gate electrodes are present on the gate dielectric which are also provided with a silicon nitride top layer.
From a technological point of view, it is advantageous if windows are also formed in the top layer of silicon nitride present on the other gate electrodes at the location where they cross channel stop regions. These windows are formed before the insulation layer of silicon oxide is deposited. This measure enables all contact holes to all gate electrodes to be formed in one process step. The contact holes are all formed by etching in silicon oxide, so that etching in silicon oxide and silicon nitride to form some contact holes, and etching in silicon oxide only to form others, is no longer required.
In a simple, very light-sensitive image sensor, the gate electrodes are formed by juxtaposed, equally thick strips of polycrystalline silicon which are provided, at the upper side, with a top layer of silicon nitride and, at the side faces, with a layer of silicon oxide formed by thermal oxidation of polycrystalline silicon. The gate electrodes can be formed in one very thin, deposited layer of polycrystalline silicon, as a result of which the loss of light during image recording is reduced to a minimum. In addition, all gate electrodes are provided with a top layer of silicon nitride, causing light reflection to be counteracted. It has been found that such an image sensor without the above-described windows in the silicon nitride top layer present on the gate electrodes does not operate satisfactorily, which can be attributed to the fact that the above-mentioned diamond-shaped picture errors occur during image recording. Passivation of the above-mentioned surface states using hydrogen is not possible. It would be expected that hydrogen can penetrate through the silicon oxide edges of the gate electrodes and, via these electrodes and the diffusion windows, into the layer of silicon oxide of the gate dielectric. However, in practice this is not the case. In the customary manner of forming silicon oxide on the side faces of the very thin gate electrodes, whereby polycrystalline silicon is heated in water vapor, a layer having a very high silicon nitride content is formed below the silicon oxide. This is commonly referred to as the xe2x80x9cwhite ribbon effectxe2x80x9d. The layer thus formed counteracts transport of hydrogen. Passivation is possible by means of the windows in the silicon nitride top layer present on the gate electrodes.
Optimum passivation using hydrogen is obtained if all gate electrodes extend, at the location where they cross channel stop regions, into diffusion windows formed there in the silicon nitride layer of the gate dielectric and, in particular, if windows are also formed in the silicon nitride top layer on all gate electrodes, which windows are in line with the diffusion windows in the layer of silicon nitride of the gate dielectric.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.